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Steam Port Area.

681

area (A),  or of half the fly-wheel section.     Bat volume = first
moment of A

3rc^ nioment of A

D2 _    v

"~ vol.

ist moment of A

the moments being found graphically at p. 845, If inches have
been adopted, R must be changed to feet when inserting in the fly-
wheel formula. Also #must be measured at radius R. If W does
not now agree with the calculated weight, the section must be
altered, and a new calculation made.

Area of Steam Port.  Practice has decided certain average
speeds of piston in particular cases, and the following list has
been thus deduced : 

MEAN PISTON SPEED IN FEET PER MINUTE.
Locomotives .........................................  1000

Marine engines .......................................    700

Horizontal engines ....................................    400

Pumping engines   ...................................    130

To attain this speed, we must not endeavour to pass the steam
through the steam port at a greater speed (according to Rankine)
than 100 feet per second.

Ratio of

cylinder area

too

port area         speed of piston in ft. per sec.

If the port be much contracted, a lower piston speed will be
attained than that intended. We shall close this chapter with
some practical examples, together with a few remaining points
of theory thereupon raised.

Horizontal Compound Pumping Engines.  Figs.
663-4-5 are views of a pair of compound engines designed and
built by the East Ferry Road Engineering Works Company,
Millwall, serving as examples of the Stationary or Land engine.
The engines are used at the Millwall Docks for pumping water to
hydraulic accumulators, under a pressure of 750 Ibs. per sq. in.
The bed plate H is in two parts, and supports the high-pressure
cylinders BB, the low-pressure cylinders A A, the pumg cylinders
zz, and the crank shaft bearings mm. The H.P. cylinder is
supplied with a main valve D, and an expansion valve E, of Meyer
form, the valve spindles being lettered respectively G and F. The